This invention relates to composite materials and, more particularly, to repairing composite sandwich structures.
Composite structures are commonly known and used in, for example, gas turbine engines for aerodynamic fairings. One type of composite is a sandwich structure having a face skin and a back skin that are bonded to a honeycomb core. The face skin and back skin may be laminated structures with a resin matrix, such as epoxy, with one or more plies of reinforcement fibers.
Laminated skins are fabricated in a process that involves compacting the resin filled plies of reinforcing fiber together while curing the resin. The texture of the tooling materials used on each molded face of the laminate during compaction and cure will be observed on the resulting laminate surface. If one or more of the molded faces is restrained by a non-rigid material, textures may telegraph thru the thickness of the laminate. During compaction against the ends of the honeycomb locally press into the skins and form a dimpled imprint surface.
In some instances, it is desirable to locally replace a honeycomb that has become damaged from an impact, elevated temperatures, abrasion, erosion, or other phenomenon rather than replace the entire composite structure. The honeycomb is machined to have a flat surface on both sides. The flat surfaces do not conform well with uneven surfaces without crushing the honeycomb. It is unfeasible to have the ends of the replacement honeycomb align perfectly with the dimpled imprints left by the removed honeycomb section. Traditionally, the damaged honeycomb is removed and an adhesive is applied to the laminate dimpled imprint surface to bond the skin and a replacement honeycomb together. Typically, an additional thickness of adhesive is used to “absorb” the unevenness of the dimpled imprints. However, adhesive bond strength rapidly declines as thickness increases, and structural requirements will limit the simple application of extra adhesive. Thus, there is a need for a repair method that provides improved bonding between the repair honeycomb and uneven laminates. This invention addresses those needs while avoiding the shortcomings and drawbacks of the prior art.